(1) Field of the Invention
The present invention relates to a wear resisting particle and a wear resisting structure member, specifically to a wear resisting particle which can be uniformly dispersed in a molten weld pool, and also to a wear resisting structure member which is provided with a buildup layer in which the wear resisting particle are almost uniformly dispersed.
(2) Description of Related Art
There are three typical examples of conventional processes to manufacture a wear resisting structure member in which hard particles as wear resisting particles are dispersed: namely, the first one is to form a wear resisting buildup layer by forming a molten weld pool at the buildup portion using consumable electrode arc welding, tungsten inert gas welding, gas welding, plasma powder welding, and the like, while adding carbide particles as the hard particles to the molten weld pool to conduct buildup action; the second one is to insert carbide particles into the coating of the coated arc electrode in advance, or to embed carbide particles into a hollowed arc rod; and the third one is the cast-in insertion process which injects a molten metal into the mould while setting carbide particles therein.
(The Case of Arc Welding and Gas Welding)
Tungsten carbide (WC, W2C) based compounds have the highest performance among the hard particles. The tungsten carbide-based compounds, however, have larger specific gravity than any of mother materials, and the hard particles of the tungsten carbide-based compound unavoidably settle in the wear resisting buildup layer independent of the particle size, thereby resulting in cohesion in the lower layer, as illustrated in FIG. 25. Coarser particles more easily settle. As a result, the lower layer of the wear resisting buildup layer gives stronger wear resisting property, and the upper layer thereof gives weaker wear resisting property. In addition, the agglutinated portion of the hard particles likely induces cracks which easily propagate, thus readily becoming the portion of separation of the wear resisting buildup layer.
Since tungsten carbide easily dissolves in Fe, a eutectic carbide of Fe—W likely precipitates in the wear resisting buildup layer, thus becoming brittle and easily generating cracks, and gives poor impact resistance. In recent years, the price of tungsten ore has increased, and tungsten carbide becomes extremely expensive among the hard particles, giving the unit price per kg of as high as one hundred and several tens of times the price of steel sheet. That is a disadvantage of tungsten carbide in terms of cost, which only allows being used in limited applications. Since tungsten carbide easily dissolves in Fe, a brittle compound is likely to be formed at interface between the hard particle and the mother phase metal. Accordingly, the important points on forming a wear resisting buildup layer in which tungsten carbide is dispersed are that the hard particles are not heated as far as possible, and that the contact time between the hard particles and the molten weld pool is shortened. Even when the tungsten carbide is eluted into the mother phase metal, the mother phase metal is hardened to an adequate level, thus to improve the wear resisting property if only the amount of the eluted tungsten carbide is at an adequate level. A long time period of heating may allow the Fe atoms to penetrate into the carbide, which results in alteration of the hard particles, thus significantly deteriorating the hardness.
Since chromium carbide (Cr3C2) is an inexpensive material, it is a kind of hard particles applied in a largest amount. However, chromium carbide has lower specific gravity than that of Fe. As a result, it floats on the molten weld pool to result in cohesion in the upper layer, as illustrated in FIG. 26. In addition, since chromium carbide is readily dissolved in Fe, coarse and non-melted hard particles are not easily retained, which deteriorates the wear resisting property of the wear resisting buildup layer in some cases.
Titanium carbide (TiC) or titanium carbonitride (TiCN) is accepted to give excellent wear resisting property next to tungsten carbide (WC), and gives high hardness and thermal stability, thus little reacting with Fe. Therefore, TiC and TiCN have an advantage of allowing to be easily remained as non-melted high hardness and high toughness particles in the wear resisting buildup layer. They are, however, low in specific gravity so that they likely float on the molten weld pool and tend to distribute only in the surface layer of the wear resisting buildup layer, as illustrated in FIG. 27. Since non-melted and coarse hard particles increase the buoyancy, they likely float. In addition, since TiC or TiCN has poor wettability, it shows weak bonding force with the mother phase metal in some cases. When mild steel is used as the mother phase metal, the TiC component is eluted very little so that the mother phase metal does not harden, and deteriorates the wear resisting property.
(The Case of Coated Arc Electrode)
Adding to the heating of arc electrode by Joule's heat, the hard particles are directly exposed to the arc, thus the dissolving of hard particles becomes significant, and the non-melted hard particles are difficult to remain. When TiC hard particles are used, TiC has small reactivity with Fe, and is thermally stable. However, since large amounts of TiC are discharged as slag, TiC does not effectively function for improving the wear resisting property in some cases. The non-uniform distribution of not-melted particles caused by the difference of specific gravity between the TiC particles and the mother phase metal occurs similar to the above case.
(The Case of Cast-in Insertion)
Since the hard particles having different specific gravity from each other have to be fixed, they are forcibly fixed to the cast utilizing wire mesh, water glass, or the like. However, to the pressure at the injection of molten metal, that type of physical fixation is not sufficient, and the arrangement of hard particles is lost in some cases. For the case of cast-in insertion, the hard particles are exposed to the molten metal for a long period of time, thus elution often occurs. In this regard, the TiC-based compounds are advantageous because they are thermally stable and do not easily react with Fe.
FIG. 28 is a schematic drawing illustrating another conventional method for manufacturing the wear resisting structure member. The manufacturing method aims to solve the problem of non-uniform distribution of hard particles owing to the difference in specific gravity between the hard particles and the mother phase metal.
The mechanism for forming the buildup layer, illustrated in FIG. 28, forms the wear resisting buildup layer on a mother material 2. According to the mechanism, an arc electrode 1 made of a welding wire projecting by 25 mm in length is positioned aslope at a tilt angle θ1 (torch angle of 30°) to the right-angle direction of the mother material 2 made of mild steel horizontally positioned. The arc electrode 1 is operated at 280 A of welding current and 28 V of welding voltage, with 100 g/min of feed speed of the welding wire, while supplying carbon dioxide as the shield gas at 30 liter/min into the welding zone. To a molten weld pool 3 formed by an arc generated from the arc electrode 1, there are supplied hard particles 4 composed of WC-7% Co particles (14.5 g/cm3 of density) having 1.2 mm in particle size and second particles 5 composed of steel balls (7.8 g/cm3 of density) having 1.7 mm in particle size via a bifurcate nozzle 6. The bifurcate nozzle 6 weaves (at 30 mm in amplitude of vibration) driven by triangular waves of 1.5 Hz in the direction of welding, or in the direction of this side to far side of the drawing of FIG. 28, thereby feeding the hard particles 4 and the second particles 5 at a rate of 172 g and 28 g per minute, respectively, (at a volume mixing ratio of 1:0.3).
The welding progresses at a speed of 22 cm per minute to the right-hand in FIG. 28 under the above conditions. The molten metal in the molten weld pool 3 supplied with the hard particles 4 and the second particles 5 has a density ranging from 7.06 to 7.21 g/cm3.
As shown in FIG. 28, both the hard particles 4 and the second particles 5 are supplied at rear side (left side), in the direction of welding progress, from the position where the straight line extending the arc electrode 1 crosses the plane of the surface of the mother material 2. Since the molten metal portion in the molten weld pool 3 at the position of supplying these particles 4 and 5 is pushed up by the action of arc, the molten metal portion is solidified without allowing the hard particles 4 to settle. In addition, during the push-up movement, the hard particles 4 and the second particles 5 are mixed together, thus a buildup layer 7 formed by hardening the molten metal portion contains uniformly-dispersed hard particles 4. As a result, the buildup layer 7 has a favorable wear resisting property, (for example, refer to Patent Document 1).    [Patent Document 1] Japanese Patent Laid-Open No. 8-47774 (from paragraph 39 to paragraph 41, and FIG. 2)